Photochemical and solar degradation of pharmaceuticals in water

Active pharmaceutical ingredients (APIs), despite their beneficial effects on human or animal health, have emerged as an environmental pollutant due to their increased use and subsequent continuous entry into the aquatic environment. Existing wastewater treatment technologies are not designed to handle this specific class of pollutants and inadequate removal is commonly observed. Even in trace levels, APIs can trigger deleterious effects in terrestrial and aquatic organisms. This has resulted in APIs being recognized as an increasingly important environmental pollutant. This concern has necessitated the search for effective technologies to deal with this continuous accumulation of APIs in the environment. The application of photochemical treatments, namely homogeneous and heterogeneous photocatalysis based on ultraviolet (UV) and sunlight, for the removal of APIs in water has been highlighted. Of these, titanium dioxide (TiO₂) photocatalysis has emerged as an effective treatment methodology for pharmaceutical removal. Its widespread application on large scales has been hindered by unfavourable kinetics, low degrees of mineralization and also high costs related to the use of artificial light and the recovery of TiO₂ nanoparticles. These issues need to be addressed now to ensure its adaptation for water treatment in the near future. The aim of this study was to investigate the effects of direct photolysis and TiO₂ photocatalysis in combination with sunlight, a sustainable and low-cost light source and also UV light for the degradation of diclofenac (DCF), naproxen (NPX) and their mixtures in different water matrices. The efficiency of immobilized TiO₂ was furthermore assessed for amoxicillin (AMX) degradation. In most cases, direct photolysis by UV irradiation allowed complete degradation for individual and mixture APIs. Lower degradation rates were observed when drinking water and river water were used compared to distilled water suggesting that the water matrix affects the efficiency of this process. The role of UV/TiO₂ photocatalysis in degrading DCF and NPX were examined by varying experimental parameters such as concentrations of APIs, TiO₂ loadings, solution pH and water matrices, with all parameters influencing the degradation of the APIs to some extent. A significant relationship between water matrix and the effectiveness of TiO₂ photocatalytic process has been observed. For mineralization, UV/TiO₂ photocatalysis led to higher oxidation rates compared to direct photolysis, although overall mineralization rates were incomplete. Studies using DCF and NPX mixtures revealed that degradation of NPX was slowed down in most cases in the presence of DCF. The degradation rates of both, DCF and NPX, in drinking water were suppressed in the presence of anions. Solar degradation studies showed comparable performances with those undertaken under laboratory conditions although longer exposure times were generally required for the degradation of APIs. A dependency of the removal efficiency on the water matrix was also found. Although differing sunlight intensities somehow impacted on the performances of solar photolysis and solar TiO₂ photocatalysis, the results showed that sunlight can be used as an inexpensive source of photons for API degradation. Various degradation products were identified following the TiO₂ photocatalytic treatment of DCF, NPX and their mixtures by liquid-chromatography mass spectrometry (LC-MS) and fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR-MS). The degradation of DCF and NPX produced a total of eight degradation products, mainly as a result of decarboxylation and hydroxylation. Degradation of DCF and NPX mixtures produced fifteen degradants corresponding to degradations of the individual APIs, while two degradation products with much higher molecular weight than the parent APIs were identified. Integrated photocatalytic adsorbents (IPA) were prepared from TiO₂, synthesized by a sol-gel method and natural zeolite, characterized and used for AMX degradation. The prepared IPA material from pre-treated acid-alkali zeolite calcined at 300°C under nitrogen optimally degraded AMX. The superior performance of this IPA material was confirmed by scanning electron microscope (SEM), energy-dispersive spectroscopy (EDS) and x-ray diffraction (XRD). SEM analysis showed an uneven surface structure as a result of TiO₂ cluster deposition. This increase in the surface roughness caused an increase in surface area, which then provides more active sites for adsorption and degradation to occur. XRD results showed that peaks from anatase, which are known to have a higher photocatalytic activity, were prominent in the IPA material calcined at 300°C. EDS analyses also confirmed the presence of higher amounts of TiO₂ in this material. The overall performance of the IPA material to degrade AMX was attributed to the adsorption capability of both the zeolite and photocatalytic activity of TiO₂. The immobilization of the synthesized TiO₂ on the surface of zeolite did not have a deleterious effect on the photoactivity of TiO₂. This study confirms the findings from previous studies that the water matrices have a large effect on the performance of direct photolysis and TiO₂ photocatalysis for APIs degradation. TiO₂ photocatalysis can be efficient in degrading APIs; however, its performance is influenced by various operating parameters. The results obtained from TiO₂ photocatalytic degradations of API mixtures suggest that individual API components can hinder the degradation efficiency of other ones. This effect may be more pronounced when numerous APIs are present in real wastewater. Utilization of the same reactor for both solar and laboratory studies allowed a reasonable comparison although the "photon source" significantly contributed to the degradation efficiency. UV/TiO₂ photocatalysis, while efficiently degrading parent APIs, resulted in the formation of numerous degradation products. Thus, attention needs to be paid when applying this advance technology for real wastewater treatment as these degradants may be persistent or toxic themselves. Immobilized TiO₂ on natural zeolite can be used for the degradation of APIs. In addition, it can be recovered and reused for subsequent degradations. This study has clearly demonstrated that TiO₂ photocatalysis can be applied to degrade APIs in both surface and drinking water and offers an attractive option for small-scale pharmaceutical water treatment. The complex nature of real effluents with co-existing pollutants and higher levels of organic and inorganic matter however calls for coupling of biological processes as pre- or post-treatment to improve their biodegradability